This invention relates to an electronic temperature-compensated circuit which is useful for controlled current discharge while inductive loads are being driven, and particularly to such a circuit which is integrated monolithically.
Within the scope of this invention, the problems connected with inductive loads as used in motor vehicle fuel injection control devices will be considered by way of example. In such devices, the driving of the current flow through an inductance in the injector allows the injector to open and close. In such a case, as the inductive load is being driven, it is very important that the control current flowing through the load be cut off within the shortest possible time, in order for the injector closing time to be made short and the amount of fuel injected accurately metered out.
It is recognized that, in general, the driving of inductive loads (such as solenoids, motors, or flyback inductors) brings about some problems during the transients. In fact, upon cutting off the current flowing through an inductor, a voltage surge (positive or negative overvoltage) immediately begins to occur at the inductor. That overvoltage is due to the induced electromotive force which resists any change in the current from the value attained during the "on" period, i.e. during the charging phase.
In the instance of fuel injection control in a motor vehicle, as herein considered, the load has a terminal at a fixed supply potential, so that the potential increase will occur at the node between the load element and the driver circuit. Accordingly, the amplitude of the voltage peaks at said node must be limited, since otherwise such peaks may cause breakage of the junctions in the semiconductor elements provided within the driver or other circuits connected to that same inductive load. In addition, with monolithically intergrated circuits, such peaks may be a triggering cause of parasitic transistors.
A state-of-art approach provides for the introduction, between the driver circuit and the inductive load, of circuitry including power elements for the controlled discharge of current from the load, so that the energy stored within the inductive load can be dissipated. The voltage at the load, after attaining a predetermined maximum, will remain constant for a time and then drop back to zero along with the current, following full discharge through the power elements themselves. The discharge time is therefore dependent on the maximum voltage value attained at the inductor. In such prior approaches, therefore, the circuit discharge also serves to limit the voltage rise to a predetermined maximum value. This voltage limiting effect is referred to as clamping in technical literature.
Where the current quenching time through the inductive load must be accurately controlled, this is accomplished using a current discharge circuit which is dimensioned so as to allow a suitable maximum voltage value to be selected, since, as mentioned, the duration of the discharge phase depends directly on the maximum voltage attained at the inductor upon turning off the driver circuit. It is desirable that the adjustment can be performed irrespective of outside conditions, in particular independently of temperature. A standard approach has been heretofore to use, for current discharge, the same power transistor--whether of the field-effect (usually MOS) type or the bipolar type--as is used for driving the load. Auxiliary control circuit arrangements control the voltage value at the load and automatically turn on the power element upon that voltage attaining its predetermined maximum value. Two main circuit solutions have been proposed.
A first one consists of connecting one or more Zener diodes between the control terminal of the driver power transistor and the inductive load. That is, the Zener diodes are arranged to set a maximum voltage value VD at the inductive load, because as that set value--which is equal to the sum of the Zener voltages plus the transistor's threshold voltage--is exceeded, the diodes will begin to conduct, thereby enabling conduction through the transistor until the inductor is discharged. A disadvantage of this prior circuit is that a peak voltage value exactly equal to the desired value at the inductor cannot always be obtained, because it would be dependent on the sum of the discrete Zener values, and optimum clamping cannot be achieved. Furthermore, since the voltage drops across the Zener diodes and at the control terminal of the power transistor depend on temperature in different degrees, compensation can only be obtained for certain definite values of VD.
In an attempt to obviate such drawbacks, the second known solution provides a combination of diodes, Zener diodes, and transistors in the current discharge circuit. One of the last-mentioned circuit arrangements is depicted in FIG. 1. The circuit in FIG. 1 operates as follows.
Upon the driver circuit C for the power element turning off the transistor T, an overvoltage is generated at node D. As this voltage reaches a value VD=VZ+VBE+V'BE+VGS(where, VZ is the Zener voltage of Z1, VBE is the voltage drop between the base and the emitter of transistor T1, V'BE is the voltage across diode D1, and VGS is the voltage drop between the gate and the source of transistor T when this is a field-effect transistor), the Zener diode begins to conduct, causing the transistor T to be turned back on. Thus, current discharge from the inductive load is started.
It should be noted that Z1, T1 and D1 in the Figure may represent a number N of Zener diodes, transistors and diodes connected in series, whereby VD is the result of several potential drops combined. In that way the discharge time of the inductive load can be optimized. That circuit has, however, a disadvantage in that the voltage value at the node D is not fully independent of temperature changes nor of the selected value of VD, although it does provide a broader range of temperature independence than the previously mentioned conventional solution.
It is an advantage of the disclosed invention to provide a current discharge circuit whose operation and threshold voltage are essentially independent of temperature. A further advantage is that the temperature-independence is retained as the maximum voltage value at any inductive load varies.
The disclosed circuits are particularly advantageous in automotive applications, e.g. for driving an injection coil. In such applications temperature-independence is particularly necessary to accurately control the timing and duration of fuel injection, and thereby reduce pollution.